Time domain reflectometry (TDR) may include an analysis of a conductor or other line (e.g., wire, twisted pair, coaxial cable, fiber optic line) by sending a pulsed signal into the line and then examining the reflection of that transmitted pulse. TDR may be useful in a variety of different applications. For example, TDR may be used to measure the length of a line or loop based on the delay of the reflection, or to measure the quality of the line, such as by examining the amplitude or other qualities of the reflection. TDR may be used, for example, to qualify or test a line for the suitability of Digital Subscriber Line (DSL) or cable communications, etc.
When TDR is performed, unwanted signal echoes can sometimes be a problem. For example, in many cases, the same port may be used both to transmit the pulse onto the line and to receive the reflected pulse from the line. In some cases, due to a reflection from the line, one of the paths that the transmitted pulse may travel is from the transmitter directly to the local receiver. This may create an echo (or unwanted reflection) that may be, in some cases, received almost immediately (e.g., 1–10 microseconds) after the transmission of the pulse, due to the very short path. This echo or unwanted reflection typically does not characterize the line, and therefore, may not be helpful in the TDR analysis of the line or loop. This echo or unwanted reflection which may begin near (e.g., just after) the time of pulse transmission, or near time zero, may be referred to herein as zero-echo (e.g., the echo substantially near time zero).
Due to the short path, a zero-echo may often have a relatively large amplitude and may even saturate the receiver. As a result, the zero-echo may create a blind time interval during which the reflectometer may not be able to measure any other reflections. Depending on the configuration, many other types of unwanted echoes or unwanted reflections may occur as well.
FIG. 1 is a diagram illustrating a conventional reflectometer. The reflectometer may send a pulse from pulse transmitter 105 to the loop (or line) 115 and measures the reflection with the local receiver 110. The receiver may receive the pulse that travels the path ABCBD, which is the desired reflection to be received (e.g., to analyze the loop or line). The receiver may also receive a pulse that travels the path ABD, which may be referred to as zero-echo. If a reflected signal substantially arrives at the receiver before the zero-echo has dissipated or died out, the two reflections may merge and typically cannot be separated. This may create a blind time interval during which a conventional reflectometer may not be able to measure reflections. Therefore, a need may exist for an improved reflectometer.